- Email: [email protected]
Diode laser measurements of collisional broadening in the ν5 band of C2H2 perturbed by O2 and N2
JOURNAL
OF MOLECULAR
SPECTROSCOPY
136,86-92
(1989)
Diode Laser Measurements of Collisional Broadening in the v5 Band of C2H2 Perturbed by O2 and N2 DANIEL LAMBOT AND GHISLAIN BLANQUET Laboratoire de SpectroscopicMokdaire, Fact&s UniversitairesNotre-Dame de la Paix, Rue de Bruxelles, 61, B-5000-Namur, Belgium
AND JEAN-PIERRE BOUANICH Laboratoire d ‘Infrarouge,associe’au CNRS, Bdtiment 350, Universite’de Paris&d, F-91405 Orsay Cedex, France
02- and N2-broadeningcoefficientsfor a number of acetylene lines in the P and R branches of the v5band have been measured at room temperature with a tunable diode laser spectrometer. From these results, air-broadening coefficients have been deduced, using the additivity property of the broadening coefficient. 0 1989 Academic Press, Inc. INTRODUCTION
There has been considerable interest in the acetylene molecule which has been observed in the atmosphere of the planets Jupiter and Saturn and Saturn’s satellite Titan, and which is produced by photodissociation of methane ( 1-3). Acetylene is also a trace constituent of the earth’s atmosphere and could play a nonnegligible role in global climate, in the not so far future (I, 2). Indeed, acetylene is a strong IR absorber in the 13.6 pm region ( v5 fundamental band), and the use of automobiles is increasing its concentration in the atmosphere. In this paper we present accurate measurements of linewidths at room temperature for acetylene collision broadened by O2 and N2, the two principal components of our atmosphere. A tunable diode laser (TDL) spectrometer has been used to measure collisional broadening for 17 lines with J ranging from 1 to 29 in the P and R branches of the v5 band of C2 H2 perturbed by N2, and for 2 1 lines with J ranging from 2 to 33 in the case of 02-broadening. To our knowledge, no data are available in the literature for oxygen-broadening of acetylene, while nitrogen-broadening has been measured in various regions (including 13.6 pm) by Varanasi et al. (1, 2), Podolske et al. (4), and Devi et al. (5). More recent measurements of N2-broadening have been reported by Blass and Chin (3) for a few lines in the P and Q branches of the v5 band. EXPERIMENTAL
DETAILS
The spectra were recorded using a commercial diode laser spectrometer (Spectra Physics-Model LS3), with a collimating lens replaced by a parabolic mirror. The 0022-2852189 $3.00 Copyright
0
1989 by Academic
All rights of reproduction
86 F’ress. Inc.
in any form reserved.
02- AND N2-BROADENING
IN THE v5 BAND OF CSHZ
87
spectrometer is interfaced to an HPlOOOF minicomputer. A complete description of the interface has been given previously (6). The sample was contained in a Whitecell, with 1 m between mirrors and a chosen path length of 4 m for the weaker lines (.I < 4 and J 2 20). A 40-cm Pyrex cell has been used for the stronger lines (4 < J < 20). Acetylene was supplied by Air Products and has a stated purity of 99.6%. Oxygen and nitrogen were supplied by L’Air Liquide with purities of 99.995 and 99.9%, respectively. The sample gas pressure was measured using I- and lOOO-mbar MKS Baratron manometers. The partial pressure of C2H2 ranged from 10 -3 to 10 -’ mbar depending on the line studied and was small enough to reduce the self-broadening contribution to a negligible value (at these pressures, the self-broadened linewidth is less than 2 X 10 -5 cm -I ) . We always used four pressures of the perturbing gas, ranging from 50 to 120 mbar. All the spectra were recorded at 297 + 2 K. Accurate line positions were obtained from Hietanen and Kauppinen ( 7) or from Kauppinen et al. ( private communication ) . For the relative calibration of the spectra, an air-spaced Fabry-Perot etalon, with a free spectral range of 0.029851 cm-‘, was introduced in the laser beam. Spectral
FIG. 1. Example of spectra obtained for the P( 10) line of C2H2 perturbed by N2 at 60 mbar. ( 1) Broadened line of C2H2 diluted in 60 mbar of N2; (2) continuum recorded with 60 mbar of Nz ( 100% transmission); (3) saturated reference line (0% transmission); (4) Fabry-Perot etalon fringes.
88
LAMBOT, BLANQUET, AND BOUANICH
purity of the laser modes was checked by observing the smoothness of the etalon fringe pattern and by recording a saturated line of Cz Hz. DATA REDUCTION
AND RESULTS
The study of the broadening of one C2H2 line required 12 consecutive records: (a) A record of the empty cell, which represents the true profile of the laser emission. (b) A record of acetylene at very low pressure, which yields the Doppler line convolved with the apparatus function. (c) A record of the line saturated, giving the 0% transmission level; this saturated line was obtained by increasing the gas pressure to about 1 mbar. (d) Four spectra with the same partial pressure of acetylene and four different pressures of the perturbing gas. (e) Four spectra with only the perturber, at the same pressures as previously. These spectra were required for a better baseline-location, because small variations of the baseline were observed in presence of the perturbing gas. ( f) A record of the etalon fringe pattern with the absorption cell evacuated.
Figure 1 shows an example of the spectra obtained for the P( IO) Lineof Cl& broadened by 60 mbar of NZ. The observed radiation intensity is given by the Beer-Lambert law Zt(a) =
Z0(~W
k(aV,
where Z,(u) and Zt( u) are the incident and transmitted radiation intensities at wavenumber u, I is the absorption path length, and k(u) is the absorption coefficient at
*O1
0.00
0.02
0.06
0.04
0.06
P (atm) FIG. 2. Collisional linewidth vs perturber pressure for the P( 16 ) line of CZHz broadened by N2 and Oz.
02- AND
N2-BROADENING
IN THE v5 BAND
OF CzH2
89
wavenumber u. Using this relation, the observed linewidth 2y,,, can be found directly from the spectra at a transmission T(u) = m where u. is the wavenumber at the center of the line. To determine the collisional linewidth 2y,, corrections have been applied for the effects of instrumental and Doppler broadenings. The instrumental linewidth 2y,, was determined by using the low pressure spectrum of &Hz. In this spectrum, collisional broadening was negligible and the observed profile was purely Gaussian. So, assuming a Gaussian apparatus function (8), we deduced its linewidth, using the relation
2Yapp=
ti2%td2 - (2rD)23
where 27, is the Doppler linewidth, obtained from 27, = 7.16 X 10-7ao
TABLE I N2-, 02-, and Air-Broadening Coefficients yo ( 10m3cm-’ atm-‘),
WAVENUMBER (CM-I) Iml
R
P
C2H2 + Nz R
P
m the vg Band of C2Hz at 297 K
TC air
CzH2
:2H2
+ 02
mooth.
P
mooth.
1 2 3 4 2 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
731.50&1 733.8598 736.2130 738.5636 740.9146 743.2639 745.6135 747.9640 750.3106 752.6580 755.0062 757.3511 759.6959 762.0396 764.3827 766.7241 769.0654 771.4048 773.7444 776.0822 778.4183 780.7531 783.0868 785.4191 787.7506 790.0803 792.4083 794.7345 797.0597 799.3835 801.7049 804.M50 806.3437 808.6601
724.4482 722.0948 719.7389 717.3861 715.0305 712.6757 710.3202 707.9657 705.6104 703.2556 700.9005 698.5436 696.1895 693.8339 691.4800 689.1254 686.7701 684.4164 682.0631 679.7094 677.3566 675.0040 672.6519 670.3OLHl 667.9489 665.5980 663.2479 660.8979 658.5488 656.2013 653.8539 651.5072 649.1619
139 02.0
04.2
91.6 91.7 86.2 83.6 84.1 82.5 80.3 80.0 76.8 75.0 71.7
72.8
62.1
52.7
16.1 09.4 03.7 99.1 95.3 92.2 89.7 87.7 86.1 847 83.6 82.6 81.7 80.8 79.8 78.7 77.5 76.1 74.5 72.8 70.8 68.7 66.7 641 61.7 59.3 57.0 54.8 52.8
80.2
69.2 69.8 67.5 63.1 61.1 60.7 60.6 59.7 59.9
57.8
55.4
56.0
52.6
50.2 48.4 47.7 46.0 42.8 41.2 40.7
90.3 84.8 80.2 76.3 73.1 70.5 68.3 66.5 65.0 63.8 62.7 61.8 61.0 60.3 59.5 58.8 58.0 57.2 56.3 55.4 54.4 53.3 52.1 51.0 49.7 48.5 47.3 46.0 44.8 43.7 42.7 41.8 41.0 40.4
10.7 04.2 98.8 94.3 90.6 87.6 85.2 83.2 81.7 80.3 79.2 78.2 77.4 76.5 75.5 74.5 73.4 72.1 70.7 69.1 67.4 65.5 63.6 61.3 59.2 57.0 55.0 53.0 51.1
Note. The wavenumbers of the unperturbed lines were measured by Hietanen and Kauppinen ( 7).
LAMBOT,
90
BLANQUET,
AND
BOUANICH
where T is the absolute temperature of the gas in Kelvins, 60 the wavenumber of the line in cm-‘, and M the molecular mass of C2H2 in g mole-‘. For the lines under study, Doppler linewidths range from 1.6 to 2.0 lop3 cm-‘, and the instrumental linewidth from 0.5 to 1.0 X 10m3cm-‘. The Voigt linewidth 2yv was obtained after a small instrumental correction, using Deltour’s tables ( 9). The collisional linewidth was then determined from the Voigt linewidth using the following convenient polynomial derived from the empirical relation of Oliver0 and Longbothum (IO), y = 1 + 0.00206 1x - 1.099474x2 + 0.066206x3 + 0.03 12 1x4, where y = yc/yv and x = yo/ yv. This polynomial, which is valid for any x value (0, 1 ), yields the same y values for x G 0.4 as a cubic polynomial used previously ( 1 I ). This polynomial is also more accurate than the expression derived by Wilkerson et al. ( 12). A typical plot of collisional linewidth 27, versus the perturber pressure P is shown in Fig. 2. The straight lines obtained are a verification of the proportionality of yc with P. The half-slope of these lines gives, for each line considered, the pressure broadening coefficient yo, expressed in cm-’ atm-’ (Table I). The main sources of error arise from the baseline-location and the nonlinear tuning of the laser, which can be seen from the variation of the Fabry-Perot etalon fringe spacing. The relative error in y. is estimated to be less than 5%. Within this uncertainty, y. is only 1m 1 dependent (m = -J for P(J) lines and m = J + 1 for R(J) lines), i.e., no significant difference
140
0
5
10
15
20
25
30
35
Iml FIG. 3. NZ- and 02-broadening
-,
coefficients y0 for v5 lines of C2Hz at 297 K. +, R branch; 0, P branch; Smooth curve fitting the experimental data.
02- AND Nz-BROADENING
IN THE Q BAND OF CzHz
91
is observed between P(J) and R( J - 1) lines. These data are smoothed by using a least-square procedure, and as shown in Fig. 3, the scattering of the data around an average curve is quite small. The air-broadening coefficients have been calculated from smoothed values of O2and Nz-broadening coefficients by using the relation 70 (air) = 0.797, (N2) + 0.21~~ (0,). Finally, we compare our results with previous results published for N2-broadening in the v5 and ( v4 + v5) bands of acetylene (Table II). The results are given for R and P branches except for Ref. ( 1)) where only the I m I dependence is indicated. Varanasi et al. ( 1) used a lower resolution Fourier transform spectrometer and derived a cubic polynomial in m for the self- and Nz-broadening coefficients. These results at 296 K are notably larger than ours for 1m ] < 13 and smaller at larger I m 1 values. Using a TDL spectrometer, Devi et al. (5) obtained in the (v4 + v5) band at 296 K a quite satisfactory agreement with our results; the recent measurements of Blass and Chin (3)) in the vs band, are significantly smaller. Good agreement is also observed with the results obtained by Podolske et al. (4) in the ( v4 + VS)band at 296 K. In conclusion, we have used a diode laser spectrometer to measure collisional broadenings and have obtained two coherent sets of data for the v5 band of acetylene broadened by N2 and Oz. From these data, we have derived the air-broadening coefficients of CZHZ lines. A next step of this work would be to compare our results with theoretical predictions.
TABLE II Comparison between Some Experimental Values of y0 (in IO-’ cm-’ atm-‘) for C2HZ-N2 PREVIOUS RESULTS
OUR RESULTS Id
R
2
109
3
102.0
8
86.2
P
83.6
13
82.5
14
80.3 71.7
29
111.7 *
101.7(29) d
110.3 a
86.6(11) d
99.9 a
85.5(9) d
94.5 a
84.2(26) c
71.7(7) d 52.7
P
104.7(78) c 106.9(58) d 104.2
10
20
R
107.2(23) d 81.27 b
97.3(15)‘J 85.3(12) d 84.1(26) d
84.5 a
72.66 b
82.2 a
69.46 b
81.8(12)J
68.1 a 35.2 L
53.3(6) d
Note.Our results are given with a relative error &%. For some experimental values, the assumed uncertainty in units of the last digits is given in parentheses. a Reference ( I ) ’ Reference ( 3 ) ’ Reference (4) d Reference ( 5 ) .
92
LAMBOT,
BLANQUET,
AND
BOUANICH
ACKNOWLEDGMENT Support by “Accords Culturels” between France and Belgium is gratefully acknowledged. RECEIVED:
November 23, 1988 REFERENCES
1. P. VARANASI, L. P. GIVER, AND F. P. J. VALERO, J. Quant. Spectrosc. Radial. Transfer 30, 491-504 (1983). 2. P. VARANASI, L. P. GIVER, AND F. P. J. VALERO, J. Quant. Spectrosc. Radiat. Transfer 30, 505-509 (1983). 3. W. E. BLASSAND V. W. L. CHIN, J. Quant. Spectrosc. Radiat. Transfer 38, 185-188 (1987). 4. J. R. PODOLSKE,M. LOEWENSTEIN, AND P. VARANASI, J. Mol. Spectrosc. 107,241-249 ( 1984). 5. V. MALATHY DEVI, D. C. BENNER,C. P. RINSLAND, M. A. H. SMITH, AND B. D. SIDNEY, J. Mol. Spectrosc. 114,49-53 (1985). G. BLANQUETAND J. WALRAND, Comput. Enhanced Spectrosc. 2, 135-140 (1984). J. HIETANENAND J. KAUPPINEN,Mol. Phys. 42,41 l-423 ( 198 1). A. MOUCHET, graduate dissertation, Facultts Universitaires N-D de la Paix, Namur, Belgium, 1983. J. DELTOUR, Infrared Phys. 9, 125-l 35 ( 1969). J. J. OLIVEROAND R. L. LONGBOTHUM,J. Quant. Spectrosc. Radiat. Transfer 17, 233-236 ( 1911). A. MOUCHET,G. BLANQUET,PH. HERBIN,J. WALRAND, C. P. COURTOY,AND J.-P. BOUANICH,Canad. J. Phys. 63,527-531 (1985). 12. T. D. WILKERSON,G. SCHWEMMER,B. GENTRY, AND L. P. GIVER, J. Quant. Spectrosc. Radiat. Transfer 22,315-331 (1974).
6. 7. 8. 9. 10. 11.
OF MOLECULAR
SPECTROSCOPY
136,86-92
(1989)
Diode Laser Measurements of Collisional Broadening in the v5 Band of C2H2 Perturbed by O2 and N2 DANIEL LAMBOT AND GHISLAIN BLANQUET Laboratoire de SpectroscopicMokdaire, Fact&s UniversitairesNotre-Dame de la Paix, Rue de Bruxelles, 61, B-5000-Namur, Belgium
AND JEAN-PIERRE BOUANICH Laboratoire d ‘Infrarouge,associe’au CNRS, Bdtiment 350, Universite’de Paris&d, F-91405 Orsay Cedex, France
02- and N2-broadeningcoefficientsfor a number of acetylene lines in the P and R branches of the v5band have been measured at room temperature with a tunable diode laser spectrometer. From these results, air-broadening coefficients have been deduced, using the additivity property of the broadening coefficient. 0 1989 Academic Press, Inc. INTRODUCTION
There has been considerable interest in the acetylene molecule which has been observed in the atmosphere of the planets Jupiter and Saturn and Saturn’s satellite Titan, and which is produced by photodissociation of methane ( 1-3). Acetylene is also a trace constituent of the earth’s atmosphere and could play a nonnegligible role in global climate, in the not so far future (I, 2). Indeed, acetylene is a strong IR absorber in the 13.6 pm region ( v5 fundamental band), and the use of automobiles is increasing its concentration in the atmosphere. In this paper we present accurate measurements of linewidths at room temperature for acetylene collision broadened by O2 and N2, the two principal components of our atmosphere. A tunable diode laser (TDL) spectrometer has been used to measure collisional broadening for 17 lines with J ranging from 1 to 29 in the P and R branches of the v5 band of C2 H2 perturbed by N2, and for 2 1 lines with J ranging from 2 to 33 in the case of 02-broadening. To our knowledge, no data are available in the literature for oxygen-broadening of acetylene, while nitrogen-broadening has been measured in various regions (including 13.6 pm) by Varanasi et al. (1, 2), Podolske et al. (4), and Devi et al. (5). More recent measurements of N2-broadening have been reported by Blass and Chin (3) for a few lines in the P and Q branches of the v5 band. EXPERIMENTAL
DETAILS
The spectra were recorded using a commercial diode laser spectrometer (Spectra Physics-Model LS3), with a collimating lens replaced by a parabolic mirror. The 0022-2852189 $3.00 Copyright
0
1989 by Academic
All rights of reproduction
86 F’ress. Inc.
in any form reserved.
02- AND N2-BROADENING
IN THE v5 BAND OF CSHZ
87
spectrometer is interfaced to an HPlOOOF minicomputer. A complete description of the interface has been given previously (6). The sample was contained in a Whitecell, with 1 m between mirrors and a chosen path length of 4 m for the weaker lines (.I < 4 and J 2 20). A 40-cm Pyrex cell has been used for the stronger lines (4 < J < 20). Acetylene was supplied by Air Products and has a stated purity of 99.6%. Oxygen and nitrogen were supplied by L’Air Liquide with purities of 99.995 and 99.9%, respectively. The sample gas pressure was measured using I- and lOOO-mbar MKS Baratron manometers. The partial pressure of C2H2 ranged from 10 -3 to 10 -’ mbar depending on the line studied and was small enough to reduce the self-broadening contribution to a negligible value (at these pressures, the self-broadened linewidth is less than 2 X 10 -5 cm -I ) . We always used four pressures of the perturbing gas, ranging from 50 to 120 mbar. All the spectra were recorded at 297 + 2 K. Accurate line positions were obtained from Hietanen and Kauppinen ( 7) or from Kauppinen et al. ( private communication ) . For the relative calibration of the spectra, an air-spaced Fabry-Perot etalon, with a free spectral range of 0.029851 cm-‘, was introduced in the laser beam. Spectral
FIG. 1. Example of spectra obtained for the P( 10) line of C2H2 perturbed by N2 at 60 mbar. ( 1) Broadened line of C2H2 diluted in 60 mbar of N2; (2) continuum recorded with 60 mbar of Nz ( 100% transmission); (3) saturated reference line (0% transmission); (4) Fabry-Perot etalon fringes.
88
LAMBOT, BLANQUET, AND BOUANICH
purity of the laser modes was checked by observing the smoothness of the etalon fringe pattern and by recording a saturated line of Cz Hz. DATA REDUCTION
AND RESULTS
The study of the broadening of one C2H2 line required 12 consecutive records: (a) A record of the empty cell, which represents the true profile of the laser emission. (b) A record of acetylene at very low pressure, which yields the Doppler line convolved with the apparatus function. (c) A record of the line saturated, giving the 0% transmission level; this saturated line was obtained by increasing the gas pressure to about 1 mbar. (d) Four spectra with the same partial pressure of acetylene and four different pressures of the perturbing gas. (e) Four spectra with only the perturber, at the same pressures as previously. These spectra were required for a better baseline-location, because small variations of the baseline were observed in presence of the perturbing gas. ( f) A record of the etalon fringe pattern with the absorption cell evacuated.
Figure 1 shows an example of the spectra obtained for the P( IO) Lineof Cl& broadened by 60 mbar of NZ. The observed radiation intensity is given by the Beer-Lambert law Zt(a) =
Z0(~W
k(aV,
where Z,(u) and Zt( u) are the incident and transmitted radiation intensities at wavenumber u, I is the absorption path length, and k(u) is the absorption coefficient at
*O1
0.00
0.02
0.06
0.04
0.06
P (atm) FIG. 2. Collisional linewidth vs perturber pressure for the P( 16 ) line of CZHz broadened by N2 and Oz.
02- AND
N2-BROADENING
IN THE v5 BAND
OF CzH2
89
wavenumber u. Using this relation, the observed linewidth 2y,,, can be found directly from the spectra at a transmission T(u) = m where u. is the wavenumber at the center of the line. To determine the collisional linewidth 2y,, corrections have been applied for the effects of instrumental and Doppler broadenings. The instrumental linewidth 2y,, was determined by using the low pressure spectrum of &Hz. In this spectrum, collisional broadening was negligible and the observed profile was purely Gaussian. So, assuming a Gaussian apparatus function (8), we deduced its linewidth, using the relation
2Yapp=
ti2%td2 - (2rD)23
where 27, is the Doppler linewidth, obtained from 27, = 7.16 X 10-7ao
TABLE I N2-, 02-, and Air-Broadening Coefficients yo ( 10m3cm-’ atm-‘),
WAVENUMBER (CM-I) Iml
R
P
C2H2 + Nz R
P
m the vg Band of C2Hz at 297 K
TC air
CzH2
:2H2
+ 02
mooth.
P
mooth.
1 2 3 4 2 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
731.50&1 733.8598 736.2130 738.5636 740.9146 743.2639 745.6135 747.9640 750.3106 752.6580 755.0062 757.3511 759.6959 762.0396 764.3827 766.7241 769.0654 771.4048 773.7444 776.0822 778.4183 780.7531 783.0868 785.4191 787.7506 790.0803 792.4083 794.7345 797.0597 799.3835 801.7049 804.M50 806.3437 808.6601
724.4482 722.0948 719.7389 717.3861 715.0305 712.6757 710.3202 707.9657 705.6104 703.2556 700.9005 698.5436 696.1895 693.8339 691.4800 689.1254 686.7701 684.4164 682.0631 679.7094 677.3566 675.0040 672.6519 670.3OLHl 667.9489 665.5980 663.2479 660.8979 658.5488 656.2013 653.8539 651.5072 649.1619
139 02.0
04.2
91.6 91.7 86.2 83.6 84.1 82.5 80.3 80.0 76.8 75.0 71.7
72.8
62.1
52.7
16.1 09.4 03.7 99.1 95.3 92.2 89.7 87.7 86.1 847 83.6 82.6 81.7 80.8 79.8 78.7 77.5 76.1 74.5 72.8 70.8 68.7 66.7 641 61.7 59.3 57.0 54.8 52.8
80.2
69.2 69.8 67.5 63.1 61.1 60.7 60.6 59.7 59.9
57.8
55.4
56.0
52.6
50.2 48.4 47.7 46.0 42.8 41.2 40.7
90.3 84.8 80.2 76.3 73.1 70.5 68.3 66.5 65.0 63.8 62.7 61.8 61.0 60.3 59.5 58.8 58.0 57.2 56.3 55.4 54.4 53.3 52.1 51.0 49.7 48.5 47.3 46.0 44.8 43.7 42.7 41.8 41.0 40.4
10.7 04.2 98.8 94.3 90.6 87.6 85.2 83.2 81.7 80.3 79.2 78.2 77.4 76.5 75.5 74.5 73.4 72.1 70.7 69.1 67.4 65.5 63.6 61.3 59.2 57.0 55.0 53.0 51.1
Note. The wavenumbers of the unperturbed lines were measured by Hietanen and Kauppinen ( 7).
LAMBOT,
90
BLANQUET,
AND
BOUANICH
where T is the absolute temperature of the gas in Kelvins, 60 the wavenumber of the line in cm-‘, and M the molecular mass of C2H2 in g mole-‘. For the lines under study, Doppler linewidths range from 1.6 to 2.0 lop3 cm-‘, and the instrumental linewidth from 0.5 to 1.0 X 10m3cm-‘. The Voigt linewidth 2yv was obtained after a small instrumental correction, using Deltour’s tables ( 9). The collisional linewidth was then determined from the Voigt linewidth using the following convenient polynomial derived from the empirical relation of Oliver0 and Longbothum (IO), y = 1 + 0.00206 1x - 1.099474x2 + 0.066206x3 + 0.03 12 1x4, where y = yc/yv and x = yo/ yv. This polynomial, which is valid for any x value (0, 1 ), yields the same y values for x G 0.4 as a cubic polynomial used previously ( 1 I ). This polynomial is also more accurate than the expression derived by Wilkerson et al. ( 12). A typical plot of collisional linewidth 27, versus the perturber pressure P is shown in Fig. 2. The straight lines obtained are a verification of the proportionality of yc with P. The half-slope of these lines gives, for each line considered, the pressure broadening coefficient yo, expressed in cm-’ atm-’ (Table I). The main sources of error arise from the baseline-location and the nonlinear tuning of the laser, which can be seen from the variation of the Fabry-Perot etalon fringe spacing. The relative error in y. is estimated to be less than 5%. Within this uncertainty, y. is only 1m 1 dependent (m = -J for P(J) lines and m = J + 1 for R(J) lines), i.e., no significant difference
140
0
5
10
15
20
25
30
35
Iml FIG. 3. NZ- and 02-broadening
-,
coefficients y0 for v5 lines of C2Hz at 297 K. +, R branch; 0, P branch; Smooth curve fitting the experimental data.
02- AND Nz-BROADENING
IN THE Q BAND OF CzHz
91
is observed between P(J) and R( J - 1) lines. These data are smoothed by using a least-square procedure, and as shown in Fig. 3, the scattering of the data around an average curve is quite small. The air-broadening coefficients have been calculated from smoothed values of O2and Nz-broadening coefficients by using the relation 70 (air) = 0.797, (N2) + 0.21~~ (0,). Finally, we compare our results with previous results published for N2-broadening in the v5 and ( v4 + v5) bands of acetylene (Table II). The results are given for R and P branches except for Ref. ( 1)) where only the I m I dependence is indicated. Varanasi et al. ( 1) used a lower resolution Fourier transform spectrometer and derived a cubic polynomial in m for the self- and Nz-broadening coefficients. These results at 296 K are notably larger than ours for 1m ] < 13 and smaller at larger I m 1 values. Using a TDL spectrometer, Devi et al. (5) obtained in the (v4 + v5) band at 296 K a quite satisfactory agreement with our results; the recent measurements of Blass and Chin (3)) in the vs band, are significantly smaller. Good agreement is also observed with the results obtained by Podolske et al. (4) in the ( v4 + VS)band at 296 K. In conclusion, we have used a diode laser spectrometer to measure collisional broadenings and have obtained two coherent sets of data for the v5 band of acetylene broadened by N2 and Oz. From these data, we have derived the air-broadening coefficients of CZHZ lines. A next step of this work would be to compare our results with theoretical predictions.
TABLE II Comparison between Some Experimental Values of y0 (in IO-’ cm-’ atm-‘) for C2HZ-N2 PREVIOUS RESULTS
OUR RESULTS Id
R
2
109
3
102.0
8
86.2
P
83.6
13
82.5
14
80.3 71.7
29
111.7 *
101.7(29) d
110.3 a
86.6(11) d
99.9 a
85.5(9) d
94.5 a
84.2(26) c
71.7(7) d 52.7
P
104.7(78) c 106.9(58) d 104.2
10
20
R
107.2(23) d 81.27 b
97.3(15)‘J 85.3(12) d 84.1(26) d
84.5 a
72.66 b
82.2 a
69.46 b
81.8(12)J
68.1 a 35.2 L
53.3(6) d
Note.Our results are given with a relative error &%. For some experimental values, the assumed uncertainty in units of the last digits is given in parentheses. a Reference ( I ) ’ Reference ( 3 ) ’ Reference (4) d Reference ( 5 ) .
92
LAMBOT,
BLANQUET,
AND
BOUANICH
ACKNOWLEDGMENT Support by “Accords Culturels” between France and Belgium is gratefully acknowledged. RECEIVED:
November 23, 1988 REFERENCES
1. P. VARANASI, L. P. GIVER, AND F. P. J. VALERO, J. Quant. Spectrosc. Radial. Transfer 30, 491-504 (1983). 2. P. VARANASI, L. P. GIVER, AND F. P. J. VALERO, J. Quant. Spectrosc. Radiat. Transfer 30, 505-509 (1983). 3. W. E. BLASSAND V. W. L. CHIN, J. Quant. Spectrosc. Radiat. Transfer 38, 185-188 (1987). 4. J. R. PODOLSKE,M. LOEWENSTEIN, AND P. VARANASI, J. Mol. Spectrosc. 107,241-249 ( 1984). 5. V. MALATHY DEVI, D. C. BENNER,C. P. RINSLAND, M. A. H. SMITH, AND B. D. SIDNEY, J. Mol. Spectrosc. 114,49-53 (1985). G. BLANQUETAND J. WALRAND, Comput. Enhanced Spectrosc. 2, 135-140 (1984). J. HIETANENAND J. KAUPPINEN,Mol. Phys. 42,41 l-423 ( 198 1). A. MOUCHET, graduate dissertation, Facultts Universitaires N-D de la Paix, Namur, Belgium, 1983. J. DELTOUR, Infrared Phys. 9, 125-l 35 ( 1969). J. J. OLIVEROAND R. L. LONGBOTHUM,J. Quant. Spectrosc. Radiat. Transfer 17, 233-236 ( 1911). A. MOUCHET,G. BLANQUET,PH. HERBIN,J. WALRAND, C. P. COURTOY,AND J.-P. BOUANICH,Canad. J. Phys. 63,527-531 (1985). 12. T. D. WILKERSON,G. SCHWEMMER,B. GENTRY, AND L. P. GIVER, J. Quant. Spectrosc. Radiat. Transfer 22,315-331 (1974).
6. 7. 8. 9. 10. 11.